1. Field of the Invention
The present invention relates to the field of ultraviolet (UV) lithography. More particularly, the present invention relates to methods and systems for performing and optimizing ultraviolet lithography, e.g. extreme ultraviolet lithography (EUVL).
2. Description of the Related Technology
In the production of today's integrated circuits, optical lithography is one of the key techniques. The ongoing miniaturization of integrated circuits or other devices has resulted in the use of new types of lithography, e.g. in lithography using electromagnetic radiation with short wavelength. A typical example thereof is extreme ultraviolet lithography.
The EUVL projection optics is composed by multiple mirrors, as shown in FIG. 1, illustrating a lithographic system 100 comprising an irradiation source 102, a mask 104 and a substrate 106. Whereas in conventional lithography techniques light typically is transmitted through the reticle, extreme ultraviolet lithography typically uses a reflective mask with a multi-layer coating. The illumination therefore typically is performed using illumination system that is non-telecentric with respect to the reticle side. In other words, illumination of the reticle typically is performed with a non-zero angle of incidence. In lithography application wherein the wavelength of the radiation used is substantially of the same order of magnitude or smaller than the thickness of the reticle features, the mask is to be approximated as a thick mask where radiation having a non-zero angle of incidence might be blocked by reticle features. In operation, EUV radiation thus illuminates the mask, causing an asymmetry of the diffracted waves because the oblique illumination (incident angle larger than zero) on a thick mask. The illumination beam is then shadowed by the edge of the absorber, and as a result, the printed patterns are shifted and/or biased. This effect is generally reported as shadowing. In addition, for large-field systems such as EUV scanners, the optics is off axis, which means that the illumination at the reticle is a curved slit.
A number of correction techniques are known for dealing with shadowing effects in extreme ultraviolet (EUV) lithography. Some solutions include the provision of a reflective layer at the top, whereby absorbing material is provided at in between the features or at the sides thereof. In US 2003/0013216 A1 a method for fabricating an EUV reflection mask is described wherein the EUV reflection mask is constructed by writing a pattern in a multiplayer on a mask. The latter results in a mask wherein no shadowing effect occurs at the light absorbing regions as their surface does not project from the surface of the multiplayer layer. In US 2002/0192571 A1, a method for fabricating a reflective lithographic mask is described wherein an absorber layer is applied on top of a substrate below a reflection layer or at the side areas of the reflection layer. In this way, shadowing effects are reduced as the absorber layer does not or less provide shadowing effects due to the configuration of the reflection layer. The methodology proposed to compensate shadowing in these cases involves heavy changes in mask topography. The latter often results in high manufacturing costs and technical complexity. Another methodology suggests to modify the aberrations in the system to compensate for shadowing, as e.g. described in US patent application 2004/0157136A1 and US2004/01376677A1, having a significant impact on the optical performance of the system.